The present invention relates to a centrifuge, in particular a continuous-flow centrifuge without face seals for centrifuging biological fluids.
Centrifuges are known in which the biological fluid is centrifuged in continuous flow. The fluid is supplied to a rotating centrifuge chamber and removed from the chamber through a line. Because of the relative movement of the centrifuge chamber and the stationary tie-in point of the line, however, guidance of the line has proven to be problematical. To prevent twisting of the line, rotating seals are used at the tie-in points of traditional continuous-flow centrifuges. Such continuous-flow centrifuges allow a high rotational speed, but the rotational couplings may lead to leakage and damage to components contained in the fluid.
A blood centrifuge without face seals is disclosed in German Patent 32 42 541 A1. With this centrifuge, which does not use face seals, the line passes from a stationary tie-in point in a loop around the centrifuge chamber. Therefore, the line is connected to a rotating frame which rotates at half the rotational speed of the centrifuge chamber. Such a continuous-flow centrifuge is known from German Patent 42 20 232 A1, for example.
In the case of continuous-flow centrifuges that do not use face seals, the line is exposed to relatively great mechanical stresses that increase greatly with an increase in rotational speed. Under the influence of centrifugal forces, the line forms a loop protruding outward, so that high reversed bending loads occur at the stationary tie-in point and the connection to the separation chamber. The steep inlet and outlet angles at the tie-in points lead to additional friction between the connection adaptors and the line, which in turn results in increased abrasion. The reversed bending load and abrasion are the factors which limit the lifetime of the line and the maximum rotational speed of the centrifuge.
Continuous-flow centrifuges that do not use face seals are known, whereby bearings are used to support the line. For example, European Patent 112 990 A1 describes a continuous-flow centrifuge in which the centrifuge tubing is supported between the stationary tie-in point and the connection of the separation chamber to two friction bearings. The friction bearings, consisting of inner and outer bearing shells, are components of the centrifuge tubing. As disposable items, the friction bearings are simple and economical to manufacture but they have relatively high friction losses, particularly at high rotational speeds.
International Patent Application WO 95/17261 A1 describes a continuous-flow centrifuge in which the centrifuge tubing is supported with a roller bearing mounted on the rotating frame. The roller bearing, consisting of the inner and outer bearing shells with the roller bodies is part of the line. Although the roller bearing offers the advantage of low bearing friction, it is complicated to manufacture and therefore the price is relatively high. This is important inasmuch as the centrifuge line is a disposable item which is discarded after use. A continuous-flow centrifuge which does not use face seals and has a bearing for supporting the fluid line is also known from German Patent 198 03 535 A1.
U.S. Pat. No. 4,221,322 describes a continuous-flow centrifuge in which the fluid line is supported on a rotationally symmetrical bearing surface having a section formed by a rotating arc. U.S. Pat. No. 4,111,356 proposes a guide element for supporting the fluid line which also has a rotationally symmetrical bearing surface.
The object of the present invention is to create a centrifuge that can be manufactured especially easily and inexpensively, whose line for supplying and/or removing the fluid is adequately supported on the one hand while on the other hand being exposed to relatively low mechanical stresses so that high rotational speeds are possible. In addition, it is especially simple and inexpensive to manufacture the line.
The bearing surface of the guide element supporting the line of the centrifuge according to the present invention is formed by a rotating planar curve, where the radius of the respective circle of curvature, which contacts the curve at a point, increases with an increase in distance between the point of contact and the central axis of rotation of the centrifuge. This yields the result that abrasion of the guide element and the line is largely uniform over the entire bearing surface. It has been found by the present invention that the lifetime of the line is improved by uniform abrasion.
The quotient of the respective radius of curvature R2 and the respective distance R1 between the contact point and the axis of rotation should be as constant as possible over the course of the curve describing the bearing surface of the guide element. The constant a=R2/R1 is greatly influenced by the materials used and the geometric boundary conditions. In particular, the flexural rigidity of the line is important. It has been found by the present invention that its lifetime can be increased particularly when 1xe2x89xa6axe2x89xa62.
In a preferred embodiment of the present invention, the guide element is a sleeve-shaped body which surrounds the line. Both the guide element and the line are preferably made of abrasion-resistant materials which can slide easily on one another without the use of a lubricant.
In an especially preferred embodiment of the present invention, the bearing surface of the guide element is connected to a cylindrical guide section whose diameter is preferably such that the line is guided loosely in it. The cylindrical guide section is used to guide a straight section of the line, while the bearing surface serves to support a bent line section downstream from the former.
To support the line, the centrifuge preferably has two guide elements. The first guide element is provided on the side of the separation unit which faces away from the stationary tie-in point. With this guide element, the line extending away from the separation unit is guided in a loop around the separation unit to the stationary tie-in point, whereupon a first section of the line is supported on the bearing surface of the guide element.
The second guide element which is provided at the stationary tie-in point is supported with its second bearing surface on a second section of the line which is guided about the separation unit.
With the bearing surfaces according to the present invention on the two guide elements, the line is supported so that it is exposed to relatively low mechanical stresses so that high rotational speeds are possible even after lengthy standing times. In principle, however, only the one guide element or the other may have the bearing surface according to the present invention.
Depending on the shape of the loop in which the line is to be guided, other guide elements may also be provided. Each guide element may also have two of the bearing surfaces according to the present invention to provide support for the line.